Natural resources beneath the surface of the earth include oil, gas, water, minerals, and sources of geothermal energy. It is a primary need of an industrialized society to locate natural resources, yet sub-surface resources or deposits may be buried at significant depths and be difficult to find. Geologists and geophysicists are typically engaged in such searches, because the different types of resources are concentrated and distributed in special relationships to the structure of the earth. Particularly, sedimentary deposits may trap resources such as oil and gas, while faulting often provides a path for migration of such resources through the earth's crust, as well as a trap for trapping the resources. It is critical to understand the underlying geology to make, essentially, educated guesses as to where certain resources may be found, and therefore where to make the often enormous investment of drilling deep into the earth.
Processes used to explore for diamonds, other gemstones, gold, silver, copper, other minerals, geothermal deposits, oil, gas, water, and other sub-surface natural resources share inherent similarities.
The first step is to identify where on the earth there are known deposits of the resources of interest. Known deposits of sub-surface resources are correlated to specific age and type of geology. This often provides the first indication of what to look for when searching in other areas. Surface geologic maps are used to identify areas in which certain deposits are more likely to be found.
For example, the best diamond mines are found in kimberlite pipes, which connect rapid violent volcanic eruptions with anomalously enriched exotic mantle compositions derived from depths of 150 to 450 km. Surface geologic maps which show the location of potassic volcanic rocks provide the clues to finding kimberlite pipes.
As another example, porphyritic intrusive rocks and fluids accompanying the transition and cooling from magma to rocks are a primary source of copper, and also typically contain molybdenum, silver, and gold. Sulfide minerals, which are economically important as metal ores, as well as iron and nickel deposits are also formed by mantle intrusions. Surface geologic maps identify trends indicating where these more electrically conductive minerals may be found.
Hot water that can be used for geothermal energy production is found near tectonic plate boundaries, such as along the California coastline, and also near mantle derived intrusive rocks that have not completely cooled, such as in Yellowstone National Park.
Oil and gas deposits typically occur in sedimentary basins and are thought to originate from (1) source rocks that have been buried deep enough to generate the high pressures and temperatures needed to crack the organic rich rock, (2) reservoir rocks through which the hydrocarbons can migrate and in which the hydrocarbons may become stored, and (3) seal rocks that trap and keep the hydrocarbons from escaping into the atmosphere. Surface geologic maps identify the trends indicating where such structural and stratigraphic traps may be found.
As an example, sediments eroded from relatively high areas such as mountains are deposited in relatively low-lying areas such as river channels, lake beds, and off-shore on the sea floor, as essentially planar, horizontal layers. Where these layers contain economically important fluids such as water, oil, methane, carbon dioxide, or other natural gases, they are referred to as “reservoir rocks.” Hot water may also be contained in a reservoir rock, where in this example the water is not economically important as a fluid, but as a stored source of geothermal energy.
Tectonic plate activity can deform these reservoir rocks and force them out of planarity, by distorting them into ridge forms referred to as anticlines that, by rising and dipping, provide pockets for entombing the natural resources stored therein. It is very common to find water in reservoir rocks, with oil above the water and gas above the oil, each fluid layer separated from the fluid below depending on relative fluid density, with all fluid layers being trapped by the anticline.
The tectonic activity may also create cracks or faults in the reservoir rocks, and as the fault blocks move on either side of a crack or fault they grind together, producing fine particles that act as a seal. Thus a fault line or crack can also act in the manner of an anticline to trap the natural resources of any reservoir rocks therein.
Tectonic activity and faults are also related to natural resources other than oil and gas. For example, faults can provide paths for the flow of hot fluids from the hotter depths of the earth, the fluids carrying minerals in solution, upwardly toward the cooler surface of the earth, the minerals crystallizing along the way at different elevations depending on temperature. Deep seated “transform faults” may carry important mineral resources, such as gold, lead, silver, zinc, and copper, up from the mantle.
These examples show, as is otherwise readily understood by geologists and geophysicists, that important natural resources are associated with particular types of geology.
Maps of topography, soils, vegetation, and water table are also useful for identifying sub-surface geology, and satellite imagery can identify patterns in soils and vegetation which relate to sub-surface geology.
Once a search area is identified by use of maps, site specific data are collected. Oil seeps, natural gas seeps, pockmarks or mud volcanoes, and reefs growing on sub-sea expressions of fault scarps may provide direct evidence of oil or gas deposits. However, the best data are typically obtained from well cores and cuttings. Cores and cuttings are studied in laboratories to determine pore size, porosity, permeability, mineralogy, lithologies, fluid content, paleo-fossils, etc. Well data are extensively used to map out sub-surface mineral deposits before starting an open pit mine or sinking shafts.
In oil and gas exploration there are numerous well logging devices that measure physical properties of the rocks adjacent to the well bore, which are critical to exploration. Such devices collect data, which are used to generate sub-surface maps of geologic horizons, lithologies, and fluids. Important measurements made with these logging devices include spontaneous potential (“SP”) and resistivity. SP logs measure the effect of telluric currents in a borehole that vary with salinity differences between the formation water and the borehole mud filtrate, and indicate permeable beds (e.g., sands and dolomites) and impermeable beds (e.g., shales and salts).
Resistivity logs measure the bulk resistivity of a formation, which is a function of porosity and pore fluid. Porous rock containing conductive saline water will of course have a lower resistivity, whereas non-porous rock or hydrocarbon filled formations have a relatively high resistivity.
Geophysical exploration makes use of a wide variety of measurement devices, such as for measuring outgassing, soil chemistry, gravity fields, electromagnetic fields, and earthquake activity, and devices used to generate and record seismic energy used to image the sub-surface.
For many years, oil and gas prospects have been evaluated with 2-D or 3-D seismic surveys. Seismologists have developed sophisticated seismic acquisition, processing and visualization tools for the prediction of lithologies and fluids from seismic reflection and refraction data.
Electromagnetic tools are used to measure and infer the electrical properties of soils and shallow bedrock, and to identify electrical conductivity contrasts between geological units. VLF methods use “back pack” portable instruments to measure local perturbations in the very low frequency (15-30 kHz) radio signals generated by the world-wide radio transmitters designed and used for submarine communications. The technique has been used for fault mapping, groundwater investigations, overburden mapping, contaminant mapping, and mineral exploration. “CSEM,” or “controlled source electromagnetics,” is another electromagnetic tool that is used in marine environments to measure the electrical resistivity associated with the presence of oil and gas.
All of these techniques are indirect tools used to infer what may lie underground, since the sub-surface cannot be directly tested without physical penetration. However, all are imperfect, and so a number of different types of techniques and tools are used in conjunction, the more so as people must search for natural resources that are becoming increasingly difficult to find. There is, therefore, always a need for another method for locating sub-surface natural resources, and there is especially a need for such a method that produces better results as presented herein.